A telephone switching system can provide a facility in which plural devices such as telephone sets or other station apparatus (referred to herein collectively as devices), can be rung using a single telephone number. For example, it is common to have a group of devices in a ring group, all of which are rung when a switching machine is put into night service. As another example, a subscriber could have devices such as a cellular wireless telephone and a telephone wired to a switching office both rung using the same subscriber number. A subscriber could call the switching office and enter a code which instructs the switching office to route all calls to his directory number, to a particularly identified subscriber's line.
In all the cases noted above, a telephone switching machine must route incoming calls to the subscriber's directory number, to a particularly designated subscriber's line or to a group of such lines, either ringing all or ringing them in a set order. This has been found to be inefficient, because ringing all devices needlessly wastes ringing current and can disturb others within hearing range of the ringing devices. If the devices are rung in sequence, they can delay pickup of the call, because the subscriber to whom the call is directed could be located at the last device in the sequence, and would not know the call is to be picked up until the ringing reaches that last device.